空指针异常在获取manyToOne的对象时

Question

我有两个表Stock和StockLocation。因为每个股票都有多个StockLocation，而每个StockLocation只有一个股票。

当我从StockLocation获取库存时，我得到了空指针异常

 @Entity(name = "stock")
 public class Stock {
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private Long id;

   @OneToMany(cascade = CascadeType.ALL, mappedBy="stock")
   @JsonManagedReference
   private List<StockLocation> stockLocations;
 }

 @Entity(name = "stock_location")
 public class StockLocation {

   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private Long id;

   @ManyToOne(fetch=FetchType.EAGER) 
   @JoinColumn(name = "stock_id")
   @JsonBackReference
   private Stock stock;
}

现在，当我尝试从StockLocation获取库存时，将出现Null指针异常。

StockLocation sl = stockLocRep.findById(id).get();
sl.getStock();